using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {

            string[] str = Console.ReadLine().Split(' ');//2つ以上のスペース区切り入力の取得

            string[] ncen = new string[11];
            for (int i = 0; i < 9; i++)
            {
                ncen[i] = str[i];
            }
            for(int i = 0; i < 9;i++)
            {
                if((i+1) != int.Parse(ncen[i]))
                {
                    Console.WriteLine(i+1);
                    break;
                }
            }
        }
    }
}